1. Field of the Invention
This invention relates generally to the use of proportional counters for neutron detection during borehole logging. In particular, this invention relates to an apparatus and method for the suppression of proportional counter microphonic noise produced by high amplitude mechanical shocks inherent in the process of borehole drilling.
2. Description of the Related Art
The use of proportional counters for neutron detection in borehole logging-while-drilling is made extremely difficult by high amplitude mechanical shocks inherent in the borehole drilling environment. Typically, such shocks cause mechanical displacements of the components within the proportional counters as well as mechanical displacements of the electronic components attached to the proportional counters. When there are component movements within a proportional counter, modulation of the effective capacitance of the detector in the proportional counter occurs. This modulation, coupled with the high voltage bias applied to the proportional counter, causes a current to flow that produces a signal in the electronic preamplifier of the counter. Such signal is generally referred to as microphonic noise. If the magnitude of the microphonically produced current is significant, the signal in the form of a voltage pulse, produced at the preamplifier's output will exceed the threshold of the voltage pulse discriminator connected to the preamplifier output. This results in microphonic noise being mistakenly counted as a neutron detection event.
In a typical logging-while-drilling neutron measurement, arrays of proportional counters are placed at various distances from the neutron source. The ratios of the count rates from these arrays are related to the properties of the formation surrounding the borehole. The set of counters farthest from the borehole may record only a few tens of counts per second (cps). Because of the low count rates, the counters farthest from the source are particularly susceptible to false neutron counts caused by additive microphonic noise. Just a few extra counts per second in these counters from microphonic noise can significantly bias the determination of geological information computed from counter array ratios.
U.S. Pat. No. 5,023,450 which issued Jun. 11, 1991 describes the problem of microphonic generation of noise in thermal neutron detectors used in measuring-while-drilling (MWD) systems. The '450 patent, incorporated herein for its description of the environmental forces applied to neutron detectors in a measuring-while-drilling system, describes a circuit for common mode rejection of noise. The '450 patent illustrates two detectors provided in a drill collar to detect neutron count rate. The output of one detector is connected to the positive input of a differential amplifier. The output of the second detector is connected to the negative input of such differential amplifier. Common vibration induced signals of the two detectors are canceled at the output of the differential amplifier.
The '450 patent disclosure assumes that neutrons returning from the earth formations surrounding the borehole will impinge on the two detectors at different times. Accordingly, the output of the differential amplifier produces a pulse stream representative of neutrons striking the two detectors.
In practice however, noise pulses do contaminate the output stream of the differential amplifier. The '450 patent disclosure provides no means for identifying noise pulses of such output stream and eliminating them from a pulse count in order to further reduce micro-phonic noise from a measured neutron count rate.
An object of this invention is to provide a method and electronic apparatus for suppressing proportional counter microphonic noise generated during borehole logging-while-drilling operations.